1. Field of the Invention
The invention generally relates to a system for remotely repairing or reconstituting nuclear fuel rod assemblies in the spent fuel pool of a nuclear power plant. The system may be installed and operated in spent fuel pools of widely varying dimensions and water levels, and is capable of repairing and reconstituting all different sizes and types of fuel rod assemblies.
2. Description of the Prior Art
Tools for repairing the fuel rod assemblies used in nuclear power plants are known in the prior art. Such tools may be used to either reconstitute a fuel rod assembly having damaged fuel rods, or to reassemble a fuel assembly having a damaged support skeleton. In reconstitution-type repairs, only the defective fuel rods from the undamaged support skeleton are pulled and replaced with new fuel rods. In reassembly-type repairs, all of the undamaged fuel rods are removed from the damaged support skeleton and inserted into a new, undamaged skeleton. In order to implement these two types of repair operations, both manual and computer-operated tooling systems have been developed for individually gripping, lifting, lowering, and ungripping specific fuel rods from one support skeleton to another.
Unfortunately, the use of such tools is not without shortcomings. How ever, before these shortcomings may be fully appreciated, some brief background as to the structure, operation and environment of such fuel rod assemblies is necessary.
Nuclear fuel rod assemblies generally comprise between two hundred and two-hundred-and-ninety fuel rods mounted in a square array within a support skeleton. The support skeleton in turn is formed from bottom and top nozzles which are interconnected to one another by twenty-four uniformly arrayed thimble tubes. The bottom and top nozzles are eight to nine inches square, and the thimble tubes are about thirteen feet long, so that the overall shape of the fuel assembly is that of an elongated, rectangular prism (see FIG. 1). The fuel rods themselves are about twelve feet long. In order to equidistantly space the long and relatively flimsy fuel rods within the support skeleton, the skeleton includes approximately seven grids, each of which has a square array of open cells for receiving and spacing the fuel rods. The grids are usually formed from flat plates of interlocking sheet metal in an "egg crate" configuration which lends compressive strength to the grids with a minimum of weight. In operation, an array of fuel rod assemblies is lowered into the reactor core by a crane, the control rods of the fuel assemblies are removed, and a jet of pressurized water is guided through the bottom nozzles thereof in order to uniformly absorb the heat generated by the rods. Typically, the velocity of the pressurized water forced through the bottom nozzles of the fuel structures is on the order of fifteen feet per second.
In some nuclear cores, this fifteen feet per second flow of water has created pressure differentials which in turn have resulted in side currents that flow laterally through the fuel rod assemblies disposed in the core. These side currents sometimes produce vibrations in the fuel rods which can eventually weaken and break the rods through a fretting action. Additionally, the support skeletons that hold the fuel rods can become damaged as a result of routine handling of the fuel assemblies.
In order to repair such damaged fuel rod assemblies, the damaged assembly is typically lowered into the cask-loading area (or shaft) of the spent fuel pool of the nuclear plant. The cask-loading shaft is approximately forty feet long, and filled with water in order to shield workers (who typically stand on a deck located over the shaft) from radiation.
In manual reconstitution or repair operations, the workers on the deck over the cask-loading shaft use elongated hand tools capable of gripping and withdrawing a single rod out of the fuel assembly standing on the bottom of the shaft after the top nozzle has been cut and removed therefrom. Small television cameras are often mounted on these tools so that the workers may visually position them over a particular fuel rod. While such tools are capable of gripping, lifting, lowering and ungripping either damaged or undamaged fuel rods within a support skeleton, they are also long and flimsy, and hence slow and cumbersome to use. While the water in the cask-loading shaft does afford an effective shield for the majority of radiation emanating from the fuel rod assembly being repaired, the workers positioned on the deck still receive some amount of potentially hazardous radiation largely due to the length of time necessary to complete such a completely manual repair or reconstitution operation.
In order to solve the problems associated with such manual repair and reconstitution tools, a computer-controlled repair and reconstitution system was developed by the Westinghouse Electric Corporation. This system is described and claimed in U.S. patent application Ser. No. 746,897 filed June 20, 1985, by Anoop Kapoor et al. and assigned to the Westinghouse Elevator Company, the entire specification of which is incorporated herein by reference. While this system represents a substantial advance in the art and solved many of the problems associated with the previously described manual tooling systems, it, too, is not without certain limitations. For example, this particular system is complicated, and requires at least four technicians in order to be operated. Additionally, this particular system is not easily adaptable to fuel pools of different heights and water levels. Finally, this system requires a substantial number of parts which must be manufactured and assembled to within close tolerances.
Clearly, there is a need for an improved fuel reconstitution and repair system that is simpler and less expensive to manufacture, install and operate than prior art systems. Ideally, the components of such a system should be adjustable to accommodate the different sizes and water levels of different spent fuel pools, and operable by only a very few technicians in order to minimize the cost of operation. Finally, it would be desirable if such a system were easily transported, assembled and installed on various sites, highly reliable, and fast in operation so as to minimize the amount of radiation exposure the system operators receive.